1. Field of the Invention
The present invention principally relates to a radio that performs transmission of data between equipment, and, more particularly, to a receiver and transmitter-receiver well suited for applications, such as a pager, a cordless remote control, a cordless telephone and a telemeter, that involve repeating intermittent reception or switching of receiving channels.
2. Description of the Related Art
In recent years, communication data transmission using a radio of the type that performs intermittent reception and transmission or frequently repeats switching of transmitting and receiving channels for effective use of both limited radio wave resources and power sources is largely used in, for example, a pager, a cordless remote control, a cordless telephone and a telemeter.
Such a conventional transmitter-receiver of the type that performs intermittent reception or frequently repeats switching of several receiving channels will be described by means of the drawings. FIG. 5 is a block diagram showing the configuration of a conventional transmitter-receiver. An antenna 100 to which a radio-frequency signal is input is coupled to the common contact 102a of a switch 102, and a receiving contact 102b of the switch 102 is coupled through a radio-frequency filter 104 composed of a band-pass filter such as an SAW or LC filter for rejecting signals in an unnecessary band to a receiving radio-frequency amplifier 106. The output of the receiving radio-frequency amplifier 106 is coupled to a frequency converter 108, to which the output of a PLL synthesizer 110 is also coupled. The output of the frequency converter 108 is coupled through an intermediate frequency filter 122 that rejects signals in an unnecessary band as in the radio-frequency filter 104 and a demodulation circuit 124 to a demodulation data output terminal 126.
In a PLL synthesizer 110, the output of a voltage control oscillator (VCO) 116 is input to a variable frequency divider 118. The variable frequency divider 118 divides a frequency of an input signal by N for output. A frequency division ratio is then preset to a predetermined value. The output of the variable frequency divider 118 is input to a phase comparator 112 that performs phase comparison with the output of a reference signal source 120 and outputs a signal depending on the magnitude of the phase difference, wherein the output of the phase comparator 112 is input through a low-pass filter 114 that rejects radio-frequency components to a control terminal of the voltage control oscillator 116. If a frequency of the reference signal source 120 is selected as a channel interval frequency, a frequency of the voltage control oscillator 116 may be varied by one channel each time a frequency division number of the variable frequency divider 118 is varied by 1. Thus setting a frequency division number respectively may output a frequency corresponding to any channel frequency. A crystal oscillator may be used as a reference signal source 120 or a frequency divided signal of the output of a crystal oscillator may be used for radio-frequency stability.
A modulation data input terminal 128 to which modulation data is input is coupled through a modulation signal generator 130 to the PLL synthesizer 110, and the output of a modulation signal generator 130 is also coupled through the reference signal source 120 to the PLL synthesizer 110. The output of the PLL synthesizer 110 is coupled through a transmission radio-frequency amplifier 132 to the transmission contact 102c of the switch 102.
Operation of the transmitter-receiver thus configured during reception will now be described. A radio-frequency signal input to the antenna 100 passes from the common contact 102a of the switch 102 through receiving contact 102b to a radio-frequency filter 104 where signals in an unnecessary band are rejected and amplified by the receiving radio-frequency amplifier 106 to input to the frequency converter 108. On the other hand, the output frequency of the PLL synthesizer 110 is set to a somewhat higher or lower frequency with respect to a radio-frequency signal to be received. The output of the PLL synthesizer 110 is input to the frequency converter 108 where it is mixed to output an intermediate frequency signal which is a difference between the frequencies, signals in an unnecessary band are rejected by an intermediate frequency filter 122 before demodulation is performed by demodulation circuit 124, demodulation data is output from the demodulation data output terminal 126.
Operation during transmission will now be described. The modulation data input to the modulation data input terminal 128 is input to the modulation signal generator 130 where the PLL synthesizer 110 is modulated according to the modulation data, and a signal is generated for modulating a frequency of the PLL synthesizer 110 through the reference signal source 120. The modulation output of the PLL synthesizer 110 thus modulated is amplified by the transmission radio-frequency amplifier 132 and passed through the common contact 102a of the switch 102 that is switched to the transmission contact 102c to be output from the antenna 100.
However, a problem with such a conventional receiver using a PLL synthesizer 110 is that the use of a feedback loop for control to obtain a predetermined frequency requires some time for boot-up operation of powering and frequency switching. Boot-up time and switching time of frequency are characterized by a natural angular frequency of the feedback loop of the PLL. Setting a large natural angular frequency may reduce time required in boot-up operation of powering and frequency switching. However, due to the constraints of setting a signal at a channel interval corresponding to each channel and that of C/N or spurious characteristics, it is difficult to have a sufficiently large natural angular frequency. Therefore, the comparison frequency of the phase comparator must be equal to the channel frequency interval or less due to the constraint of setting a signal at a channel interval corresponding to each channel, large loop gain may not be obtained. For this reason, a conventional receiver using a PLL synthesizer has limits to reduce boot-up operation time and frequency switching time and sufficient characteristics may not be obtained to meet these demands.
Since a pager, a cordless remote control, a cordless telephone and a telemeter often involve repeating intermittent transmission and reception or switching of transmitting and receiving channels and employ a battery as a power source, however, boot-up time of frequency switching will become longer, leading to reduced battery life and lowered commercial value.